How to Configure EIGRP Summarization in Cisco?

Pre-requisite: EIGRP Configuration, EIGRP Implementation in Cisco.

With respect to EIGRP or any Interior Gateway Protocol (IGP), route summarization is a significant tool to create a boundary for query propagation and is needed to create a single route entry with a prefix & prefix length larger than one or smaller component routes.

Route summarization reduces the amount of routing information that routers must exchange, process, and maintain, which ultimately allows for faster topology convergence and less CPU utilization of the router within the network. Summarization also restricts the size of an area that is affected by network changes By hiding the changes in the individual networks behind a single advertised summary route that encompasses all the component routes.

EIGRP supports 2 types of route summarization :

1. Automatic summarization
2. Manual summarization

EIGRP Route Summarization commands :

• Automatic summarization is practically unusable in today’s networks as it is deactivated by default since the IOS release 15.0(1)M. In older IOS versions, the following command should be used in the EIGRP configuration to deactivate it :

Device(config-router)#no auto-summary

• Manual summarization allows summarizing routes at any chosen router and its specific interface in the network. Manual summarization is configured on a per-interface basis using the following command : 

Device(config-if)#ip summary-address eigrp 
autonomous-system address netmask
 [ distance] [ leak-map name ]

NOTE: leak-map argument is optional and allows referring to the route map. 

• Whenever a summary route is advertised, the router performing the summarization automatically installs a discard route for this summary route into its routing table. The subnet & net mask in this discard route is identical to the subnet & netmask of the advertised summary, and the outgoing interface is set to Null0.
• Discard route prevents suboptimal routing or routing loops when a router advertises a summary route without knowing a more specific matching subnet for ingress traffic.

Configure EIGRP Summarization in Cisco Packet Tracer:

Step 1: Create the 2 router topology in Cisco Packet Tracer as shown in the image below :

Step 2: Configure the IPv4 address on the physical interface on the routers :

R1(config)#interface FastEthernet0/0
R1(config-if)#ip address 10.0.0.1 255.255.255.0
R1(config-if)#no shutdown

R2(config)#interface FastEthernet0/0
R2(config-if)#ip address 10.0.0.2 255.255.255.0
R2(config-if)#no shutdown

Step 3: Configure a loop back interface on R2 with an IPv4 address :

R2(config)#interface Loopback0
R2(config-if)#ip address 8.8.8.8 255.255.255.255

Step 4: Configure loop back interfaces on R1 with IPv4 address :

R1(config)#interface Loopback0
R1(config-if)#ip address 172.168.1.1 255.255.255.0
R1(config-if)#interface Loopback1
R1(config-if)#ip address 172.168.2.1 255.255.255.0

Step 5: Configuring the EIGRP process on both routers with the command router eigrp <1-65535 Autonomous system number> :

Router(config)#router eigrp 1
Router(config-router)#network 10.0.0.0
Router(config-router)#network 172.168.1.0
Router(config-router)#network 172.168.2.0

Router(config)#router eigrp 1
Router(config-router)#network 10.0.0.0
Router(config-router)#network 8.8.8.8

Step 6: Verifying EIGRP configuration:

R1#show ip protocols

R2#show ip protocols  

Step 7: Checking the IPv4 routing table on the routers and seeing the EIGRP route entry :

R1#show ip route

R2#show ip route

• Two individual network/subnet route entries.

Step 8: Configuring R1 to always originate and propagate a summarized route for subnets 172.168.1.0/32,172.168.2.0 /32 throughout the entire EIGRP autonomous system :

R1(config)#int fastEthernet 0/0
R1(config-if)#ip summary-address eigrp 1 172.168.0.0 255.255.252.0

• Whenever a summary route is advertised, the router performing the summarization automatically installs a discard route for this summary route into its routing table. The subnet & net mask in this discard route is identical to the network & netmask of the advertised summary, and the outgoing interface is set to Null0.
• To verify the above, check the IPv4 routing table of R1 and see the entry :

R1#show ip route

Step 9: Verifying whether the summarized route is being propagated correctly by R1 or not, by viewing the IPv4 routing table of R2 :

R2#show ip route

Step 10: Verifying the connectivity throughout the topology

R1#ping 10.0.0.2
R1#ping 8.8.8.8

R2#ping 10.0.0.1
R2#ping 172.168.1.1
R2#ping 172.168.2.1

Result Simulation: